1. Field of the Invention
The present invention relates to an improvement in an electric power steering apparatus wherein a motor is driven by computer control based on signals from various kinds of sensors for adding a drive force of the motor to a steering system.
2. Description of the Related Art
In a conventional electric power steering apparatus, a steering torque arising from the operation of a steering wheel by a driver is detected by a steering torque sensor and a motor for providing an auxiliary torque is drive-controlled by a microcomputer based on a detected steering torque signal and signals from a vehicle speed sensor or the like. In this apparatus, there is provided only one control system having the microcomputer.
The conventional electric power steering apparatus is arranged such that when abnormality of the apparatus is detected due to failure of the vehicle speed sensor or the like, the supply of the auxiliary torque is restricted by fixing a motor current at the middle part of a vehicle-speed-responsive control range, or depending on the nature of failure of the apparatus, the function of the system per se is stopped.
However, problems arise when control of the conventional electric power steering apparatus is carried out by the single control system or when simply two control systems are provided in the apparatus.
In the electric power steering apparatus having only one control system, failure of part of the apparatus may result in stoppage of the entire apparatus. Further, the apparatus is incapable of performing control over various kinds of failures therein in correspondence with the extent of those failures.
Similarly, in the apparatus having two control systems which are independent from each other, when part of one control system (for example, one sensor) fails, the entire control system fails to function, thereby requiring switchover to the other control system. Further, such a switchover arrangement does not make the failure of the one control system known to a driver.